Help with a new plugin

I thinking about to create a new plugin that will add a new function in cart. In which the customer may request the addition of a QRCode in a product.
If the QRCode is added then I need to add a value to the product.

I would like to know, what is the best way to do a plugin for this.

Any tip is welcome.

Thanks.

It is not clear to me what you mean by " addition of a QRCode in a product".
Have you looked at Product Attributes?

I actually would like to add the QRCode in the purchase and not to the product.

This Qrcode will be like a message sent along with the purchase.

Then, in the end of the purchase the customer will be able to choose whether to add that Qrcode or not.

If it choose yes, the system must add a value to the total purchase.

I wonder if I could be clear.

Sorry, but my english isn't so good.